The Grendel Affair by Lisa Shearin

We're Supernatural Protection & Investigations, known as SPI. Things that go bump in the night, the monsters you thought didn't exist? We battle them and keep you safe. But some supernatural baddies are just too big to contain, even for us...
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When I moved to New York to become a world famous journalist, I never imagined that snagging a job at a seedy tabloid would change my career path from trashy reporter to undercover agent. I'm Makenna Fraser, a Seer for SPI. I can see through any disguise, shield, or spell that a paranormal pest can come up with. I track down creatures and my partner, Ian Byrne, takes them out--usually saving my skin in the process.
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Our cases are generally pretty routine, but a sickle-wielding serial killer has been prowling the city's subway tunnels. And the murderer's not human. The fiend in question, a descendant of Grendel--yes, that Grendel--shares his ancestor's hatred of parties, revelry, and drunkards. And with New Year's Eve in Times Square only two days away, we need to bag him quickly. Because if we don't find him--and the organization behind him--by midnight, our secret's out and everyone's time is up.

This book is part of the "SPI Files" series.
Here are some other books from this series: